1. Field of the Invention
The present invention relates to plasma or dry etching of semiconductor wafers, and more particularly to detecting the optimal point during the etching process at which to terminate the process (i.e. the endpoint).
2. Background of the Related Art
Plasma or dry etching of semiconductor wafers is well-known in the semiconductor processing art. The plasma or dry etch process is a process by which one or more layers of chemical species which have been either deposited or grown on the surface of a silicon wafer or other workpiece are removed or liberated from the surface of the wafer in those areas which are not protected by a layer of exposed photoresist that is impervious to the etching process.
The wafers are typically disposed in a sealed chamber the interior of which is held at a pressure which is substantially below that of the atmosphere. One or more gases are introduced into the chamber which are highly reactive with the chemical constituents of the layer to be etched when the gases are ionized. As gas ionization source (typically R. F. energy) generates a plasma consisting of chemically active and charged species (.about.10E12 cm.sup.3), whereafter the chemically active and charged species in the plasma impinge on the workpiece and react, dislodge, or otherwise cause material to be removed or volatilized from the workpiece and evacuated from the process chamber via a vacuum pumping arrangement.
As the plasma reacts with the unprotected layer, new compounds and/or species evolve from the surface of the wafer in the form of ions free radicals or volatilize gases. Certain of these species are elevated to higher energy states and emit photons as they fall back to a lower energy state. The photo emission for each of the species has a unique frequency which can be detected to indicate the presence of that species. The intensity of the photo emissions are directly proportional to the amount of the particular species that is present in the chamber.
It is critical to stop the plasma or dry etch process at the point where the layer to be etched has been completely removed from the surface and before the process begins to etch away the underlying layers. Otherwise, optimal operation of the resulting semiconductor circuits will not be realized. In fact, the integrated circuits assembled from the wafer may not operate at all.
Methods and apparatus for detecting an endpoint for plasma or dry etch processes have been implemented in the past. These methods and apparatus typically employ optical detectors and/or monochrometers to detect the optical emissions of one or more of the evolving species to determine the point at which those species either disappear from the chamber or begin to appear in the chamber; this is the point at which one layer has been etched away and/or the surface of an underlying layer has been reached. The monochrometers and/or optical detectors are coupled to a window of the chamber and they measure the intensity of light at specific frequencies to which they are tuned. The monochrometers and data detectors produce an analog voltage the amplitude of which is proportional to the concentration of a species in the chamber to which they are tuned. In a prior art method, the analog signals are converted to digital representations, processed and compared to a threshold level which, when reached or surpassed, serves to indicate an endpoint.
As the feature sizes of semiconductor processes have decreased to the sub-micron level, however, situations often exist where the amount of exposed area of a particular layer to be etched falls well below 5% of the total surface area of the wafer. Thus, the intensity of the optical emissions for a particular species becomes so low that they are lost in the noise generated by the system. The signal-to-noise ratio becomes so low that currently known methods of filtering the noise to identify the signals of interest are not sufficient to permit an appropriate endpoint of the etching process to be accurately and reliably detected. Furthermore, as the number of wafers processed by a particular etching apparatus becomes large, deposits on the window through which the optical emissions are viewed and detected further reduces the intensity of the signals of interest, thereby further exacerbating the shrinking signal-to-noise ratio.
Therefore, there is a need in the art of plasma or dry etching of semiconductor wafers for an improved method which can properly and reliably detect endpoints for plasma or dry etching processes operating under conditions where the exposed surface area of the layer to be etched on, for example, a semiconductor wafer falls well below 5% (to at least as low as 0.8%) of the total surface area of the wafer, and where "cloudy window" problems further exacerbate the low signal-to-noise ratio. This new and improved method of endpoint detection should also be readily adapted to all existing and future etching apparatus, and should be applicable to any of the various surface layers which are commonly etched using a plasma or dry etch process. Finally, the method should be capable of facilitating optimization of endpoint detection for a given etching process by providing for fine adjustment of the endpoint and the ability to select the optimal endpoint using empirical results recorded from previous process runs.